London -LRB- CNN -RRB- -- After pleas to spare the life of British aid worker Alan Henning went unanswered by the so-called Islamic State terror group , his family is `` numb with grief '' with the news that he has been beheaded .

`` It is the news we hoped we would never hear . As a family we are devastated by the news of his death . There are few words to describe how we feel at this moment , '' his wife , Barbara , said in a written statement released Saturday by the United Kingdom 's Foreign Office .

'' ... We always knew that Alan was in the most dangerous of situations but we hoped that he would return home to us . That is not to be . ''

News of Henning 's beheading was made public Friday in a video released by ISIS , which blamed the killing on the UK for joining the U.S.-lead bombing campaign against the group in Iraq and Syria . In the same video , the group threatened the life of American aid worker Peter Kassig .

UK Prime Minister David Cameron called Henning 's murder `` absolutely abhorrent '' and `` completely unforgiveable , '' saying Britain must work with its allies to defeat ISIS .

`` What we see with this organization is there 's no level of depravity to which they will not sink . No appeals made any difference , '' he said , speaking Saturday at the Prime Minister 's country house retreat , Chequers .

'' ... The fact this was a kind , gentle , compassionate and caring man who had simply gone to help others , the fact they murdered him in the way they did , shows what we are dealing with , and this is going to be our struggle there . With others , we must do everything we can to defeat this organization ''

But Cameron also warned the fight against ISIS , also known as ISIL , would not be quick .

The Prime Minister said he had held briefings with senior military and intelligence advisers and others , `` to make sure we do everything that we can , whether that is taking down these hateful videos from the Internet , whether it 's stopping preachers of hate , whether it 's prosecuting those who glorify terrorism , whether it 's working with others to defeat ISIL in the region . ''

ISIS video of 4th Westerner killing similar to others , but does n't run as long

Pleas ignored

The news of Henning 's beheading came just days after Britain joined the U.S.-led coalition against ISIS , following overwhelming approval from Parliament .

UK jets began flying reconnaissance flights over Iraq a week ago , and on Tuesday dropped their first missiles on an ISIS heavy-weapon position and an armed pickup truck in Iraq , according to the UK Defense Ministry .

The White House released a statement condemning Henning 's murder and vowing to work alongside the United Kingdom and its allies to `` degrade and ultimately destroy ISIL . ''

Henning appears to be the fourth Westerner to be beheaded on camera by ISIS .

This summer , ISIS beheaded American freelance journalists James Foley and Steven Sotloff -- showing their gruesome killings in videos posted online . ISIS then claimed its first British victim , aid worker David Haines , according to video that appeared online on September 13 .

After receiving an audio file with her husband 's voice , Henning 's wife attempted to contact those holding him . When she was unable to reach those who sent the file , she made public pleas last month to ISIS to spare her husband 's life -- an appeal joined by voices of Muslim leaders around the world .

But those pleas were ignored .

`` On behalf of the entire family , I want to thank everyone who campaigned for Alan 's release , who held vigils to pray for his safe return , and who condemned those who took him , '' she said , according to the statement .

`` Your efforts were a great support to us , and we take comfort in knowing how many people stood beside us in hoping for the best . ''

A taxi driver from near Manchester in northern England , the 47-year-old Henning was part of a team of volunteers who traveled to Syria in December 2013 to deliver aid -- food , water and medical supplies -- to people affected by the country 's devastating civil war .

He was abducted by masked gunmen the day after Christmas , other people in the aid convoy said .

`` Alan was a decent , caring human being . His interest was in the welfare of others , '' Barbara Henning said in the statement . `` He will be remembered for this and we as a family are extremely proud of him and what he achieved and the people he helped . ''

Who is the ISIS ?

` Our local and national hero '

At a ceremony at Manchester Central Mosque on Saturday , a joint statement mourning the loss of Henning was read out on behalf of mosques throughout the north of England .

`` Alan Henning was our local and national hero , '' it said . `` We will remember him as a tireless and selfless volunteer aid worker whose only concern was to help people in need .

'' ... The killing of Alan Henning was a cowardly and criminal act of appalling brutality by a group who do not represent Islam at all and in fact are an insult to the Islamic faith . ''

Some 200 mosques and organizations put their names to the joint statement , together representing about a million people , according to mosque officials .

Talha Ahmad , a spokesman for the Muslim Council of Britain , told CNN on Saturday that the organization was `` devastated and disgusted '' by Henning 's murder , which he called `` the latest round of despicable violence by ISIS . ''

The murder , according to Ahmad , confirmed that ISIS is `` not a religiously inspired organization '' but rather a militia group `` hell-bent on inflicting pain and cruelty . ''

He does not believe a very large number of people sympathize or are even prepared to tolerate the kind of attitude shown by ISIS , he said .

`` The challenge we have is to make the case again and again that however tempting , however lucrative it may appear to support a group which stands up to the Western ` bully , ' if you like , the reality is ISIS do not represent a hope , '' he said .

One heartening thing to come out of the tragedy of Henning 's death , he said , is that Muslims `` of all shades and colors and theology , have come together to condemn ISIS and we just need to keep doing that . ''

UK authorities believe at least 500 British citizens have gone to Iraq and Syria , many of them to fight with ISIS and other Islamist groups -- and that most will try to return , bringing their extremist views with them .

Cameron last month laid out new measures to tackle the threat posed by would-be UK jihadists , days after Britain raised its terror threat level from `` substantial '' to `` severe . ''

How ISIS is run

Laura Smith-Spark reported and wrote from London , and Chelsea J. Carter reported and wrote from Atlanta ; CNN 's Phillip Taylor , Karl Penhaul , Talia Kayali and Bharati Naik contributed to this report .

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Family of Alan Henning is `` devastated , '' says statement

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`` There are few words to describe how we feel at this moment ''

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Joint statement by mosques remembers Alan Henning as `` our local and national hero ''

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UK Prime Minister David Cameron vows to find those responsible for Henning 's murder